RPM Drops on IS250

Im just starting to notice even more that my is250 rpm is dropping when coming to a stop light. The engine studders a little bit and then corrects itself. Also it gets pretty bad when putting it in from drive to reverse, where it drops to about 500rpm and studders. I had the engine tsb done and they replaced the pistons, and short engine block etc. Im wondering if it is the MAF or Throttle body making it do this? It is somewhat tolerable but i feel as if the car isnt running as smoothly as it should.. Any ideas?

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